Electrostatographic toners are currently manufactured through two main routes. The so-called conventional toner powders, also known as melt-pulverized (MP) toner, are made up of a binder polymer and other ingredients, such as pigment and a charge control agent, that are melt blended on a heated roll or in an extruder. The resulting solidified blend is then ground or pulverized to form a powder. Inherent in this conventional process are certain drawbacks. For example, the binder polymer must be brittle to facilitate grinding. Improved grinding can be achieved at lower molecular weight of the polymeric binder. However, low molecular weight binders have several disadvantages; they tend to form toner/developer flakes; they promote scumming of the carrier particles that are admixed with the toner powder for electrophotographic developer compositions; their low melt elasticity increases the off-set of toner to the hot fuser rollers of the electrophotographic copying apparatus, and the glass transition temperature (Tg) of the binder polymer is difficult to control. In addition, grinding of the polymer results in a wide particle size distribution. Consequently, the yield of useful toner is lower and manufacturing cost is higher. Also the toner fines accumulate in the developer station of the copying apparatus and adversely affect the developer life.
The more recent and more desirable method for toner preparation is the chemically prepared toner (CPT) technique. Both toner particle size and size distribution can be more effectively controlled through various CPT technologies. The preparation of toner polymer powders from a preformed polymer by a chemically prepared toner process, known as the “evaporative limited coalescence” (ELC) technology, offers many advantages over the conventional grinding method of producing toner particles. In this process, polymer particles having a narrow size distribution are obtained by forming a solution of a polymer in a solvent that is immiscible with water, dispersing the solution so formed in an aqueous medium containing a solid colloidal stabilizer, and removing the solvent. The resultant particles are then isolated, washed and dried.
In the practice of this technique, polymer particles are prepared from any type of polymer that is soluble in a solvent that is immiscible with water. The size and size distribution of the resulting particles can be predetermined and controlled by the relative quantities of the particular polymer employed, the solvent, the quantity and size of the water insoluble solid particulate suspension stabilizer, typically colloidal silica or latex, and the size to which the solvent-polymer droplets are reduced by mechanical shearing using rotor-stator type colloid mills, high pressure homogenizers, agitation etc.
Limited coalescence techniques of this type have been described in numerous patents pertaining to the preparation of electrostatic toner particles because such techniques typically result in the formation of polymer particles having a substantially uniform size distribution. Representative limited coalescence processes employed in toner preparation are described in U.S. Pat. Nos. 4,833,060 and 4,965,131 to Nair et al., incorporated herein by reference for all that they contain.
The electrostatographic toner particles obtained by either the conventional or the ELC process described above are substantially “solid,” meaning that the particle interior is essentially continuous in terms of physical state as solid. On the other hand, if there are introduced pockets of gas (or vacuum) inside the particles the toner is then “porous.”
There is a need to reduce the amount of toner applied to a substrate in the electrophotographic process (EP). Porous toner particles in the electrophotographic process can potentially reduce the toner mass in the image area. Simplistically, a toner particle with 50% porosity should require only half as much mass to accomplish the same imaging results. Hence, toner particles having an elevated porosity will lower the cost per page and decrease the stack height of the print as well. The application of porous toners provides a practical approach to reduce the cost of the print and improve the print quality.
U.S. Pat. Nos. 3,923,704; 4,339,237; 4,461,849; 4,489,174; and EP 0083188 discuss the preparation of multiple emulsions by mixing a first emulsion in a second aqueous phase to form polymer beads. These processes produce porous polymer particles having a large size distribution with little control over the porosity. This is not suitable for toner particle.
U.S. Pat. No. 7,368,212 describes a porous toner particle. However, control of particle size distribution along with the even distribution of pores throughout the particle is a problem.
US Patent Publication Numbers 2008/0176164 and 2008/0176157 disclose a method for the preparation of a porous toner particle by the steps of forming a first water-in-oil emulsion with a first aqueous phase comprising a pore stabilizing hydrocolloid dispersed in an organic phase containing a polymer; dispersing the first emulsion in a second aqueous phase to form a second emulsion by shearing in the presence of a stabilizing agent, to form droplets of the first emulsion in the second aqueous phase; and evaporating the organic solvent from the droplets to form porous toner particles. The method is based on the evaporative limited coalescence (ELC) process and produces porous particles of narrow particle size distribution and controlled porosity.
The process of obtaining porous toner particles through the ELC process, as disclosed in the above application, requires the formation of a multiple emulsion, or a water-in-oil-in-water (W1/O/W2) double emulsion, through a two-step homogenization process where the first high-shear step is needed to produce fine droplets of stabilized aqueous phase to form a first water-in-oil (W1/O) emulsion. The second homogenization step employs reduced shear to avoid destruction of the first emulsion. This is a common practice for double emulsion preparation. However, for production-scale preparation of double emulsions, the need for this first-emulsion preparation step demands additional capital equipment, more energy use, more complicated material handling, more waste due to the multiple steps, and thus higher manufacturing cost.
The high shear that is needed to form finely dispersed W1/O emulsion can lead to undesirable outcomes. Polymers of high molecular weight may be degraded by the high mechanical shear giving uncontrolled polymer composition in the final products. The high energy input also can generate heat during the homogenization, causing heat sensitive ingredients such as wax to melt and recrystallize, and temperature sensitive hydrocolloids to change form and property.
Another drawback in making a separate W1/O emulsion is that when the viscosity of the W1 phase is high, breaking down the W1 into small droplets becomes difficult. The emulsion is difficult to handle and disperse into the second water phase.